|Special Public Works Programmes - SPWP - Planting Trees - An Illustrated Technical Guide and Training Manual (ILO - UNDP, 1993, 190 p.)|
|7. Planting trees outside woodlots and forests|
Shelterbelts or windbreaks are strips of trees and other vegetation that reduce the force of the wind. They are very important in areas with frequent high winds and windblown sand. Often they consist of several rows of shrubs and trees of different heights.
Shelterbelts are most effective if they do not block the wind completely, like a wall, but force it to slow down. If the wind cannot pass through the shelterbelt at all, it will try to pass underneath it or over the obstacle. If it flows over the shelterbelt, it produces turbulence which is harmful for the crops behind the belt. If it goes underneath, the belt acts like a funnel and the wind becomes very strong. Therefore choose large trees for the centre row. On each side of this row plant one or two rows of smaller species. Outside these rows shrubs or other vegetation can be planted to make sure that the wind cannot pass underneath the belt. Plant the trees with close spacing, e.g. 1-2 m apart.
The trees need to be able to stand up to the wind and to have flexible branches and medium dense crowns. The crowns should be long and narrow rather than spreading (the Casuarina species, for example, forms ideal windbreaks). The species should preferably be able to provide by-products. A well chosen mix will not only provide shelter from the wind but will also yield fruits, firewood, etc. If Shelterbelts are planted across grass and cropland where animals are grazing, it will be advantageous to plant thorny shrubs along the edges so that the shelterbelts can provide protection from livestock, like a live fence.
The shelterbelts should be planted perpendicular (at a right angle) to the direction in which the wind usually blows. The length of the zone protected is about 10 times the height of the shelterbelts. If large areas are to be protected, parallel shelterbelts should be planted. The spacing between the shelterbelts should then be 10-20 times the height of the tree.
side view one row of shrubs three rows of trees
good shelter belt = moderately dense
shelterbelt too dense - causes turbulence